In recent years, as environmental regulations are tightened, it is necessary to reduce contents of environment pollutants discharged from an engine, and in order to solve the pollutant discharge, it is necessary to increase a combustion temperature by increasing an explosion pressure of the engine. In this way, when the explosion pressure of the engine is increased, strength of engine cylinder block and head constituting the engine needs to be increased in order to stand the explosion pressure.
A material that is currently used for the engine cylinder block and head is flake graphite cast iron to which a very small amount of ferroalloy such as chrome (Cr), copper (Cu), or tin (Sn) is added. Since the flake graphite cast iron has excellent heat conductivity and excellent damping ability and a very small amount of ferroalloy is added thereto, the flake graphite cast iron is less likely to occur chill, and has excellent castability. However, since tensile strength is about 150 to 250 MPa, there is a limitation in using the flake graphite cast iron for the engine cylinder block and head requiring an explosion pressure of more than 180 bar.
Meanwhile, the material of the engine cylinder block and head for standing the explosion pressure of more than 180 bar needs to have a high strength of about 300 MPa. To achieve this, an element such as copper (Cu) or tin (Sn) for stabilizing pearlite or an element such as chrome (Cr) or molybdenum (Mo) for prompting generation of carbide needs to be added. However, since the addition of the ferroalloy may potentially cause the occurrence of the chill, there is a problem in that the chill is highly likely to be caused in thin walled parts of engine cylinder block and head having a complicated structure.
As the related art for achieving high strength of the flake graphite cast iron, there is a method of forming MnS emulsion by controlling a using ratio between manganese (Mn) and sulfur (S) added in a molten cast iron, that is, Mn/S to be a predetermined ratio. At this time, the formed Mn/S emulsion serves to prompt generation of the nucleus of graphite and to reduce the occurrence of the chill due to the addition of the ferroalloy. Since the aforementioned method can be applied to high manganese molten cast iron having a manganese (Mn) of about 1.1 to 3.0%, the content of the manganese (Mn) needs to be used two times more than a content of manganese used in manufacturing flake graphite according to the related art. Thus, material cost may be unavodiably increased. Further, the manganese (Mn) serves to prompt a pearlite structure, and allows a cementite distance within the pearlite structure to be densed to strengthen a matrix structure. However, when a large quantity of manganese (Mn) is added, since carbide is stabilized to disturb growth of the graphite. Accordingly, when the Mn/S ratio is not controlled to be a predetermined range, the occurrence of the chill is further prompted due to the large content of the manganese. Therefore, there is a limitation in applying the flake graphite cast iron to the engine cylinder block and head having a complicated structure.
CGI (compacted graphite iron) that has excellent castability, damping ability and heat conductivity of the flake graphite cast iron and satisfies a high tensile strength of 300 MPa or more is recently applied to engine cylinder block and head having a high explosion pressure. In order to manufacture the CGI of a tensile strength of 300 MPa or more, it is necessary to use a melting material and pig iron in which a content of an impurity such as sulfur (S) or phosphorus (P) is low, and it is necessary to precisely control magnesium (Mg) which is an element of spheroidizing the graphite. However, since it is difficult to control the magnesium (Mg) and the CGI is very sensitive to changes of melting and casting conditions such as a tapping temperature and a tapping speed, it is highly likely to cause material quality deterioration of the CGI and casting defect. Further, manufacturing cost may be increased.
Moreover, since the CGI has relatively poor processibility than the flake graphite cast iron, when the engine cylinder block and head are manufactured using the CGI, it is difficult to manufacture the engine cylinder block and head in an existing processing line for the flake graphite cast iron, and it is necessary to change the processing line to a processing line for the CGI. Accordingly, enormous facility investment cost may be incurred.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.